9.2a - Determination of pH

Materials and Apparatus Required

Chemicals

• Water samples (e.g., river, lake, or tap water)
• Universal indicator solution (or pH paper)
• Distilled water (for cleaning the pH meter probe)

Apparatus

• pH meter (or pH paper with colour chart)
• Beakers (100 cm³)
• Dropping pipette
• Safety glasses

Safety Precautions

• Wear safety glasses throughout the experiment.
• Avoid contact with potentially contaminated water samples.
• Ensure the pH meter is rinsed thoroughly between different samples to prevent cross-contamination.

Method

Using pH Paper:

1. Pour a small volume of the water sample into a clean beaker.
2. Dip a strip of pH paper into the water sample and compare the resulting colour to the pH chart provided with the paper.
3. Record the pH value.

Using Universal Indicator Solution:

4. Add 2-3 drops of universal indicator solution to a small beaker of water sample and stir.
5. Compare the resulting colour to the pH colour chart and record the pH value.

Using a pH Meter:

6. Calibrate the pH meter according to the manufacturer's instructions using buffer solutions.
7. Rinse the pH probe with distilled water and place it in the water sample.
8. Allow the reading to stabilise, then record the pH value.
9. Rinse the probe with distilled water before measuring the next sample.

Repeat the procedure for each water sample being tested.

Results

Sample	pH (pH Paper)	pH (Universal Indicator)	pH (pH Meter)
River water	7.2	7.0	7.1
Lake water	6.5	6.3	6.4
Tap water	7.8	7.6	7.7

Example Questions with Answers